Clots formed in reconstituted human plasmas or from purified human fibrins were studied to assess effects of subunit cross-linking on clot strength and resistance to plasma degradation. The relative amounts of Alpha subunit and Gamma subunit ligation were varied by addition of Factor XIII to the samples. Appreciable Gamma-Gamma cross-linking always precedes detectable Alpha dimer or Alpha polymer. Non-invasive laser light scattering measurements to the shear modulus, G(t), indicates that the ligation of Gamma subunits and of Alpha subunits have quantitatively similar effects on clot strength. Since Alpha cross-linking occurs very slowly in plasma clots, it is inferred that under physiological conditions involvement of Alpha subunits in development of clot strength is probably only a secondary function. Light scattering techniques were used to study the size of particles shed from the surfaces of fibrin clots undergoing fibrinolysis. No differences could be discerned as resulting from ligation of Gamma subunit compared to ligation of Gamma and Alpha subunits.